A paper leaf such as a currency note, negotiable securities, and the like having a portion that reflects light, and a portion that transmits light, can be read using a photo sensor. For example, a reader for reading a paper leaf using a photo sensor performs a reading process in accordance with the following operations. First, spectra having different wavelengths from one another are emitted towards a paper leaf. Then, the reader detects optical information on the paper leaf by reading reflected light information from the emitted light reflected by the paper leaf and transmitted light information. Truth or false verification of currency notes, determination of the degree of deterioration during distribution and the like are performed from the detected optical information. An optical detector that provides a reader with such optical information is configured to have a lighting system that illuminates a paper leaf, an imaging system that images the reflected light or the transmitted light from a paper leaf, photoelectric conversion elements that convert imaged optical information to an electrical signal and the like. In recent years, the detection information amount to be processed in an optical detector has been increasing exponentially on the background that the demand for currency note, verification has been increasing and other reasons.